This invention is in the field of telecommunications systems, and is more specifically directed to signal processing and interface circuitry in subscriber-line modems.
The high-speed exchange of digital information between remotely located computers is now a pervasive part of modem computing in many contexts, including business, educational, and personal computer uses. It is contemplated that current and future applications of high speed data communications will continue the demand for systems and services in this field. For example, video on demand (VOD) is one area which has for some time driven the advancement of technology in the area of digital information exchanges. More recently, the rapid increase in use and popularity of the Global Internet (hereinafter, the "Internet") has further motivated research and preliminary development of systems directed to advanced communication of information between remotely located computers, particularly in effecting higher bit-rates using existing infrastructure.
One type of technology arising from the above and continuing to evolve is referred to in the art as digital subscriber line ("DSL"). DSL refers generically to a public network technology that delivers relatively high bandwidth over conventional telephone company copper wiring at limited distances. DSL has been further separated into several different categories of technologies according to a particular expected data transfer rate, the type and length of medium over which data are communicated, and schemes for encoding and decoding the comununicated data.
In each case, a DSL system may be considered as a pair of communicating modems, one of which is located at a customer site, such as a home or office computer, and the other of which is located at a network controller site, typically a telephone company central office. Within the telephone company system, this modem is connected to communicate with some type of network, often referred to as a backbone network, which is in communication with other communication paths by way of equipment such as routers or Digital Subscriber Line Access Multiplexers (DSLAMs). Through these devices, the backbone network may further communicate with dedicated information sources and with the Internet. As a result, information accessible via the backbone network, such as Internet information, may be communicated between the central office DSL modem and a customer site having its own compatible DSL modem.
Within this general system, it is also anticipated that data rates between DSL modems may be far greater than current voice modem rates. Indeed, current DSL systems being tested or projected range in rates on the order of 500 Kbps to 18 Mbps or higher. According to certain DSL techniques, the data communication rates are asymmetric, with a considerably higher data rate used for so-called downstream communication, that is from the central office to the customer site, than for upstream communication from the customer site to the central office. Most DSL technologies also do not use the whole bandwidth of the twisted pair, reserving a relatively low bandwidth channel for conventional voice telephonic communication (commonly referred to as "POTS" for "Plain Old Telephone Service"), so that voice and data communications may be simultaneously carried out over the same line.
By way of further background, examples of DSL technologies currently being developed include High-Bit-Rate Digital Subscriber Line ("HDSL"), Single-Line Digital Subscriber Line ("SDSL"), and Very-high-data-rate Digital Subscriber Line ("VDSL"). HDSL has a symmetric data transfer rate, communicating at the same speed in both upstream and downstream directions. Current perceived speeds are on the order of 1.544 Mbps of bandwidth, but require two copper twisted pairs. However, the operating range of HDSL is somewhat limited, currently to distances of approximately 12,000 feet or less, beyond which signal repeaters are required. SDSL delivers comparable symmetric data transfer speed as HDSL, but achieves these results with a single copper twisted pair which limits the range of an SDSL system to approximately 10,000 feet. Lastly, VDSL provides asymmetric data transfer rates at much higher speeds, such as on the order of 13 Mbps to 52 Mbps downstream, and 1.5 Mbps to 2.3 Mbps upstream, but only over a maximum range of 1,000 to 4,500 feet.
The most publicized DSL technology currently under development is referred to as Asymmetric Digital Subscriber Line, or "ADSL," and corresponds to ANSI standard T1E1.413. ADSL technology encompasses communication according to Discrete Multitone (DMT) modulation, and also includes frequency domain multiplexing (FDM); other modulation techniques, such as Carrierless Amplitude/Phase modulation (CAP), are also known in the art. In any case, according the current state of the art, it is contemplated that ADSL systems will communicate data over a single copper twisted pair at downstream (central office to remote modem) rates on the order of 1.5 Mbps to 6 Mbps, with upstream rates ranging from 16 kbps to 640 kbps. A particular example of the ADSL technology utilizes a downstream (central office to remote) signal bandwidth of 25 kHz to 1104 kHz, and an upstream (remote to central office) signal bandwidth of 25 kHz to 138 kHz; in this realization, echo cancellation is especially necessary at the remote modems, since its transmission bandwidth is within the bandwidth of its received traffic. In any event, because the ADSL technology enables these high bandwidths to be attained over existing twisted pair infrastructure, telephone companies are not only contemplating providing Internet access using ADSL technology, but are also considering delivering remote LAN access and VOD services using this approach.
Of course, in addition to performance considerations and to the distance over which DSL communications may be carried by conventional twisted-pair infrastructure, the cost of the modem hardware is also a significant factor in the selection of a communications technology. It is therefore contemplated that a lower data rate technology may provide high-speed data communications, with downstream data rates exceeding 1 Mbps, over existing twisted-pair networks and at cost that is competitive with conventional non-DSL modems, such as 56 k, V0.34, and ISDN modems.
Because of-the nature of DSL communications, so-called mixed-signal circuitry is required in the implementation of DSL modems, both for the central office and also for the remote subscribers, in which both analog and digital signals are handled. Conventional DSL modem designs include functions which are referred to as "analog front ends", in which operations such as digital-to-analog and analog-to-digital conversion, power amplification, and some amount of filtering (including low-pass, band-pass, and high-pass filtering) is performed. Because of the frequencies involved in DSL technology, ranging from tens of kHz to MHz frequencies, and because of the large dynamic range required in order to accommodate the wide variations in length and quality of subscriber loops, the filtering involved in conventional DSL modems is relatively complex. As a result, typical analog front end circuitry has heretofore been realized by discrete analog circuitry, using close tolerance components to eliminate manufacturing variations. The complex filter characteristics in these conventional modems, particularly in providing a filter with the steep band rejection requirements of the DSL standards, prevents effective integration of these analog filters into an integrated circuit, especially when considering that the filter characteristics must follow process variations in the digital-to-analog converters. Specifically, it is contemplated that the integration of this analog front end circuitry into mixed signal integrated circuits would require significant "trimming" of the analog filter bandwidths in order to compensate process variations.
By way of further background, it is known in the art that significant attenuation of signals can often occur over conventional twisted pair telephone lines, particularly at the frequencies used in DSL communications. As is also well known, this attenuation is exacerbated with decreasing wire gage, with increasing wire length, and with increasing signal frequency, with the severity of this attenuation being up to on the order of -100 dB in some cases. However, conventional analog-to-digital converters (ADCs) typically have a quantization noise floor which is substantially flat over frequency. As a result, signal loss can occur in the event that the higher frequency signals are attenuated to below the quantization noise floor of the receiving ADC.